Growing up in a flexible family: behavioural development and family social dynamics in captive gibbons and siamangs (Primates: Hylobatidae)

The gibbons and siamangs (family Hylobatidae) are renowned for their unusual social organisation (social monogamy) and were once thought to be inflexible in their social behaviour; however, hylobatids share many characteristics with their cousins (the great apes) that increase the benefits of, and capacity for, social flexibility. These include: large brains relative to body size, long developmental periods and long lifespans. Moreover, recent research on other family-living species reveals a complexity in social dynamics that reflects the importance of cooperation in coordinating group activities and maintaining group cohesion. Variation in social behaviour among families of hylobatids should be investigated, but detailed observation of intra-group social behaviour is difficult in wild groups. Captive family groups provide a valuable resource for investigating variation in social behaviour in this taxon. Despite the relative abundance of hylobatids in zoos, little is known about their social development and capacity for flexible behaviour. This thesis provides the first quantitative analysis of behavioural development in hylobatid families (Hylobates moloch, Nomascus leucogenys and Symphalangus syndactylus), and explores variation in behaviour both within and among individuals, relative to the age of individuals and the reproductive and social context within the group. The behaviour of 36 individuals in 13 groups (residing in Australasian Zoos) was recorded over a 5 year period from 2008 to 2012, including over 1137 focal observation hours. For some families, data from a previous study in 2005/2006 was also included, such that the data set for some groups spanned a 7 year period.

I focus on family social dynamics and address adult pair behaviour, parental roles, immature development and patterns of aggression and affiliation within the family. The presence of offspring and the reproductive status of the breeding female produce differences in the activity budgets of both males and females in captivity, but sex differences in behaviour are greater when offspring are present, and/or when mothers are pregnant or lactating. Activity budgets of group members differ in complementary ways, such that males contribute more to parental care when females contribute less, and when no siblings are present. In addition, maturity of offspring is both age-related and conditional on the social environment; developmental timing depends on whether offspring remain in their natal group and whether younger siblings are present in the group. Contrary to expectation, the expression of bonding behaviour between adult males and females does not depend on the length of time together or the female’s reproductive status. Similarly, the birth of new offspring and greater family size does not increase conflict within family groups. The flexible social roles displayed by these captive individuals indicate that cooperation may be an important driver of social behaviour in hylobatids, as in other family-living species. In combination with other sociodemographic characteristics (e.g. production of overlapping offspring and delayed dispersal), flexible coordinated social roles may allow hylobatids to pool individual efforts in territorial defence and parental care across a small, highly related social group. In this way, the social behaviour and life history of the small apes bears a remarkable resemblance to that of humans, and some family-living birds.